Prosecutors said Dr Gosnell's legitimate-sounding Women's Medical Society was in fact a "baby charnel house", in which foetal remains were left scattered around in freezers, bags and jars.

"The clinic reeked of animal urine, courtesy of the cats that were allowed to roam (and defecate) freely," a grand jury report said. "Furniture and blankets were stained with blood. Instruments were not properly sterilised.

Disposable medical supplies were not disposed of; they were reused, over and over again".

To maximise profits, Dr Gosnell and unqualified colleagues "catered to the women who couldn't get abortions elsewhere – because they were too pregnant", it was alleged.

After fixing ultrasound scans to make foetuses look smaller, the grand jury found, they gave hundreds of pregnant women labour-inducing drugs before "snipping" the foetuses they delivered.

One of the babies Dr Gosnell was convicted of murdering, known in court as "Baby Boy A", was born after 30 weeks of pregnancy and was estimated to weight about six pounds.

"He was breathing and moving when Dr Gosnell severed his spine and put the body in a plastic shoebox for disposal," it was found. "The doctor joked that this baby was so big he could "walk me to the bus stop." Baby C, another murder victim, "was moving and breathing for 20 minutes before an assistant came in and cut the spinal cord". Baby D, born in a lavatory, had been "moving and looked like it was swimming" before being killed.

Dr Gosnell was also convicted of the involuntary manslaughter of Karnamaya Mongar, a 41-year-old immigrant from Nepal, who died in 2009 after being sedated at the clinic.

Prosecutors said that before paramedics arrived, Ms Mongar's body was rearranged to appear as though she had been undergoing a "routine, safe abortion procedure".

The doctor was acquitted of the murder of another newborn, known as Baby E.

Three other counts of alleged murder had earlier been thrown out by Judge Jeffrey Minehart over a lack of evidence that the babies had been living after birth.

"It is a victory for human rights that Gosnell was found guilty on three counts of first degree murder," said a spokesman for The Pennsylvania Family Institute, an anti-abortion group. "But on four counts, our society has looked the other way".

In a statement, Planned Parenthood, itself frequently the target of anti-abortion activists, said: "The jury has rightly convicted Gosnell for his appalling crimes, ensuring no woman is victimised by him ever again." Dr Gosnell's clinic was raided by the FBI over alleged prescription drug offences in February 2010.

Prosecutors were reported to be in tears as the verdicts were read last night, while Dr Gosnell appeared shocked. Jack McMahon, his attorney, said after the hearing that his client was "disappointed and upset", but added: "A jury has spoken and we respect that verdict".

Dr Gosnell's trial will now enter a second phase, in which the jury will consider what sentence he should receive. He could receive the death penalty or several life sentences.